but it will contain lots of beasties.
By beasties you you mean 'nasties' such as vine weevil grubs,slugs & snails or do you mean 'common or garden' beasties (pardon the pun) such as worms, wood lice etc?
If you mean the latter then I see no problem I have a habit of using this type of 'used compost' for just general potting up and usually it is OK.
Having said that;
I would never use it for seed sowing or propagation I would always use new (supposedly sterile) compost!